1. Field of the Invention
The present invention relates to the removal of tin or tin alloys from substrates and, in particular, to the making of interconnect solder bumps on a wafer or other electronic device using methods such as the well-known C4NP interconnect technology in which residual tin or tin alloy is removed from the transfer molds used in the C4NP method.
2. Description of Related Art
While the method of the invention is applicable to the removal of tin or tin alloy from any substrate the invention will be described for convenience to the removal of high tin composition solder from electronic substrates and in particular from the transfer molds used in the C4NP interconnect technology method.
Forming an electronic package assembly whereby an electrical component such as an integrated circuit chip is electrically and mechanically connected to a substrate, a card, or board, another chip or another electronic part is well known in the art. This technology is generally termed surface mount technology (SMT) and has gained acceptance as the preferred means of making electronic package assemblies. The interconnect technology is commonly known as ball grid array packaging, C4 flip chip interconnect, multi-chip modules, multilayer and micro via printed wiring boards and surface mount hybrid assembly.
Multilayer ceramic and organic electronic components are typically joined to other components by metallic termination (or capture) pads on a surface of one of the electronic components to corresponding metallic termination (or capture) pads on the surface of the other component. Control Collapse Chip Connection is an interconnect technology developed by IBM as an alternative to bonding devices to substrates with discreet wires, known in the art as wirebonding. This technology is generally known as C4 technology or flip chip packaging. Broadly stated, an integrated circuit chip is mounted above a ceramic or organic substrate and pads on the chip are electrically and mechanically connected to corresponding pads on the substrate by a plurality of electrical connections such as solder bumps to form an electronically connected module. A module is typically connected to other electronic components by solder or socket type connections.
In the C4 interconnect technology a relatively small solder bump is attached to the pads on one of the components being joined, typically to the chip. The electrical and mechanical interconnects are then formed by positioning the corresponding pads on the substrate to be joined adjacent the solder bumps on the chip and reflowing the bumps at an elevated temperature. The C4 joining process is self-aligning in that the wetting action of the solder will align the chip bump pattern to the corresponding pads on the substrate.
Capture pads for C4 bumps on semiconductor wafers or on the substrate to be interconnected are well-known and are typically made by a through resist plating of Ni or Cu/Ni pads onto a Cu seed layer. It is also preferred to use a conductive barrier layer on the substrate surface and the Cu seed layer is preferably made by sputtering of Cu onto a sputtered TiW layer. Capture pads can also be fabricated by a blanket metal sputtering process with the blanket film subsequently etched without any plating process involved.
In C4 technology the solder bumps are formed directly on the capture pads of the one unit. The pads are electrically isolated from other pads by the insulating chip passivation and substrate that surrounds each pad. The substrate may be un-doped silicon (Si) or some other material. The bottom of the pad is electrically connected into the chip or substrate circuit.
A major application of C4 is in joining semiconductor microchips (integrated circuits) to chip packages. Chips usually are made in rectangular arrays on a mono-crystalline slab of silicon called a “wafer,” which is a thin disc several inches across. Many chips are formed on each wafer, and then the wafer is diced into individual chips and the chips are “packaged” in units large enough to be handled. The C4 bumps are placed on the chips while they are still joined in a wafer.
One method of forming solder bumps uses sputtering or vacuum deposition. Solder metal is evaporated in a vacuum chamber and the metal vapor coats everything in the chamber with a thin film of the evaporated metal. To form solder bumps on the substrate, the vapor is allowed to pass through holes in a metal mask held over the substrate. The solder vapor passing through the holes condenses onto the cool surface into solder bumps. This method requires a high vacuum chamber to hold the substrate, mask, and flash evaporator. Wafers are then subjected to elevated temperatures to reflow the solder into the desired spherical bump shape.
An alternative technique for making solder bumps is electrodeposition, also called electrochemical plating or electroplating. This method also uses a mask such as patterned photoresist and forms solder bumps only at the selected sites. Other methods include screening a solder paste through a mask and depositing micro-bumps in place. High temperature reflow processes are required for all of these bump-forming methods.
The C4NP transfer process is the preferred solder bump formation method and uses a mold having a plurality of openings (cavities) corresponding to the capture pads of the wafer. These openings are filled with the solder. The solder in the mold is then transferred to the capture pads of the wafer and upon heating forms solder bumps on the wafer pads. The mold openings are preferably filled with solder by an Injection Molding Solder (IMS) method as described below.
Residual tin-rich solder can remain in the mold cavities and must be removed from the mold surface and mold openings prior to refilling the mold with solder for the next usage. This has presented a problem to the industry.
Commercially available tin stripping solutions (Eliminator made by McDermid Inc. and NPS 3000 made by RBP Corp.) are typically formulated with nitric acid (HNO3) as a primary component. Since etching with HNO3 alone will result in the formation of insoluble metastannic acid (H2SnO3) which would require a subsequent chemical step to remove it from the workpiece, secondary components (ferric nitrate, ferric chloride and hydrochloric acid) are added to the etching solutions to hold the tin in solution. The typical holding capacity of these solutions is about 120 g/l. A further limitation of these solutions is that H2 gas is evolved during the tin etch process and in addition to safety precautions required in a high volume production operation, bubble formation on the tin surface being etched leads to non-uniformity in the etching rate across the entire surface if these bubbles are not freely liberated.
Another known solution in which tin is readily dissolved is concentrated hydrochloric acid (HCl). This solution suffers the same problem as the HNO3 based solutions in that H2 gas is evolved and e.g., in the case of high tin solder composition etching, insoluble intermetallic precipitates (CuSn, AgSn) form which must be removed by another process step. The bath life of a concentrated HCl solution is also severely limited due to the high vapor pressure of HCl. Both the gas evolution and the insoluble precipitate formation render these solutions more appropriate for a spray etch process as opposed to an immersion process but spray etching has a negative environmental side effect in that volatilization of the acids is increased, resulting in additional emissions. Tool materials of construction are also a major concern when processing with high concentration HCl solutions.
Cupric chloride/HCl solutions are well-known for etching copper in electronic component manufacture is shown in U.S. Publication No. US2006/0199394 published Sep. 7, 2006. A method for the regeneration of cupric etching solutions is shown in U.S. Publication No. US2006/0196761 also published Sep. 7, 2006.
Bearing in mind the deficiencies of the prior art it is an object of the present invention to improve the C4NP process and, in particular, cleaning of the solder transfer molds to remove residual high concentration tin solder from the C4NP solder molds after the solder transfer step.
It is another object of the present invention to remove tin or tin alloys from substrates such as electronic substrates.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.